Heretofore, it has been known that silicon or GaAs monocrystal is used as a semiconductor layer in an FET element, and it is put to practical use. However, a material used for the element is expensive and the process for manufacturing the element is very complicated. In addition, an area where the element can be built in is limited by a size of a wafer. For example, when an active drive element used in a wide screen liquid crystal display element is manufactured, there is a considerable restriction in costs and an area as far as the above wafer is used. Because of such restriction, as the FET element which is used as an drive element in the liquid crystal display element, a thin film transistor using amorphous silicon is put to practical use at the present. However, in the thin film transistor using amorphous silicon, it becomes more difficult to uniformly manufacture many elements on a plane surface at low costs as its display element area is increased. Thus, recently, it has been proposed that the FET element is manufactured using an organic semiconductor Among the organic semiconductor, those using .pi.-conjugated polymer are especially remarked because it has high processability, which is a characteristic of a polymer material, and its area can be easily increased (Japanese Patent Laid Open Gazette No. 62-85224).
It is thought that the .pi.-conjugated polymer, whose skeleton of a chemical structure is of a conjugate double bond or triple bond, has a band structure comprising a valence band, a conduction band and a forbidden band separating those bands, which are formed by overlapping of .pi.-electron orbits. The forbidden band of the .pi.-conjugated polymer is mostly within a range of 1 to 4 eV, which varies with a material. Therefore, the .pi.-conjugated polymer itself shows electrical conductivity of an insulator or that close to it. However, it is described that a carrier carrying an electric charge is generated by removing an electron from the valence band (oxidation) by a chemical method, an electrochemical method, a physical method or the like or by implanting (referred to as doping hereinafter) an electron into the conduction band (reduction). As a result, it is possible to vary its electrical conductivity over a large range from an insulator region to a metal region by controlling an amount of doping. The .pi.-conjugated polymer obtained by a doping of oxidation reaction is p type and it is n type in a doping of reduction reaction. This is similar to impurity addition to an inorganic semiconductor. Thus, it is possible to manufacture various semiconductor elements using the .pi.-conjugated polymer as a semiconductor material.
As the FET element in which the .pi.-conjugated polymer is used as a semiconductor, it is known that polyacethylene is used (J. Appl. Phys., Vol. 54, pp.3255, 1983). FIG. 15 is a sectional view showing a conventional FET element using polyacetylene. In this figure, reference numeral 1 designates a glass serving as a substrate, reference numeral 2 designates an aluminum film serving as a gate electrode, reference numeral 3 designates a polysiloxane film serving as an insulating film, reference numeral 4 designates a polyacetylene film serving as a semiconductor layer and reference numerals 5 and 6 designate gold films serving as a source electrode and a drain electrode, respectively.
Operation of the FET element in which polyacetylene is used as a semiconductor layer will be described. When a voltage is applied between the source electrode 5 and the drain electrode 6, a current flows between the source electrode 5 and the drain electrode 6 through the polyacethylene film 4. At this time, when a voltage is applied to the gate electrode 2 formed on the glass substrate 1 and separated from the polyacethylene film 4 by the insulating film 3, electrical conductivity of the polyacetylene film 4 can be varied by an electric field effect, so that the current between the source and drain can be controlled. The reason for this is thought that a width of a depletion layer in the polyacethylene film 4 adjacent to the insulating film 3 varies with the voltage applied to the gate electrode 2 and then a section area of a channel comprising an effective positive carrier accordingly varies. However, the current between the source and drain which can be varied with the gate voltage is considerably small in this FET element.
As another example of the FET element in which the .pi.-conjugated polymer is used as a semiconductor, it is known that poly (N-methylpyrrole) (Chem. Lett., pp. 863, 1986) and polythiophene (Appl. Phys. Lett., Vol. 49, pp. 1210, 1986) are used. FIG. 16 shows a sectional view showing an FET element in which poly (N-methylpyrrole) or polythiophene is used as a semiconductor layer. In this figure, reference numeral 3 designates silicon oxide serving as the insulating film, reference numeral 4 designates a poly (N-methylpyrrole) film or polythiophene film serving as the semiconductor layer, reference numerals 5 and 6 designate gold films serving as the source electrode and the drain electrode, respectively, reference numeral 1 designates a silicon plate serving as the substrate and also the gate electrode and reference numeral 2 designates a metal for making ohmic contact with the silicon plate 7. When poly (N-methylpyrrole) is used as the semiconductor layer, a current (electrical conductivity) flowing between the source electrode 5 and the drain electrode 6 through the semiconductor layer 4 is only slightly controlled by a gate voltage and then there is no practical value therein.
On the other hand, when polythiophene is used as the semiconductor layer, the current (electrical conductivity) flowing between the source electrode 5 and the drain electrode 6 through the semiconductor layer 4 can be modulated 100 to 1000-fold by the gate voltage. However, since polythiophene is formed by electrochemical polymerization in the prior art, it is very difficult to uniformly manufacture many FET elements at the same time.
Thus, the current between the source and drain which can be modulated by the gate voltage is too small in the FET element in which polyacethylene or poly (N-methylpyrrole) is used as the semiconductor layer. In addition, although the current between the source and drain can be largely modulated by the gate voltage in the FET element in which polythiophene is used as the semiconductor layer and also has high stability, since the FET element is manufactured by means of forming a polythiophene film directly on the element substrate by the electrochemical polymerization, it is difficult to uniformly manufacture many FET elements on a large substrate at the same time in an element manufacturing process, which is a problem in the production process.